1) Field of the Invention
The present invention relates to friction welding and, more specifically, to a tool and method for forming lap joints by friction stir welding.
2) Description of Related Art
Friction stir welding is a process in which a rotating tool, such as a pin or probe, is urged into and/or through a workpiece, e.g., to join multiple structural members of the workpiece in a solid state or to repair cracks in a workpiece. As shown in FIG. 1, the pin 12 of the tool 10 typically extends from a shoulder 14, which can be flat, concave, or otherwise contoured, and the shoulder 14 is urged against the structural member 20 so that the pin 12 is urged into the structural members 20, 22. The pin 12 is then urged through the structural members 20, 22 to form a continuous weld joint 26. For example, according to one conventional friction stir welding process for forming a lap joint 26, a friction stir welding machine (not shown) urges the pin 12 in a direction 16 and into an interface 24 defined by the two overlapping structural members 20, 22. The rotating tool 10 is then moved in a direction 18 along the interface 24. The motion of the rotating tool 10 generates frictional and deformation heating, thereby forming a region of plasticized material in the structural members 20, 22. In some cases, the tool 10 is tilted approximately 2.5° relative to the structural members 20, 22 so that the trailing edge of the shoulder 14 is thrust into the plasticized material. Upon cooling of the plasticized material, the members 20, 22 of the workpiece are joined along the weld joint 26. Friction stir welding is further described in U.S. Pat. No. 5,460,317 to Thomas et al., the entire contents of which are incorporated herein by reference.
Friction stir welding has been demonstrated to be a successful joining method for forming typical butt joints. When forming a butt joint, the pin of the tool is inserted into the workpiece to extend substantially parallel to the interface between the structural members. Thus, the pin makes contact with the workpiece throughout the interface and mixes the material at the interface. During formation of a lap joint, however, the pin is typically inserted in a direction substantially perpendicular to the interface between the structural members. That is, as illustrated in FIG. 1, the structural members 20, 22 are overlapped to form the interface 24, and the pin 12 is inserted through the first structural member 20, through the interface 24, and then through at least a part of the second structural member 22. The mixing provided by the pin 12 is often insufficient to fully disperse the material throughout the interface 24. For example, mixing can be insufficient at the “retreating side” 34 of the weld joint 26, i.e., the transverse side of the weld joint 26 where the tangential direction of the rotational motion of the pin 12 is opposite the direction 18 of the advancement of the pin 12 through the structural members 20, 22. In some cases, mixing may also be insufficient at the opposite side, the “advancing side” 32 where the tangential direction of the rotational motion of the pin 12 corresponds to the direction 18 of the advancement of the pin 12. If the interface 24 is not sufficiently mixed, oxide layers that normally exist on the overlapping, or faying, surfaces of the structural members 20, 22 can still exist in a continuous, albeit deformed, configuration after the joint 26 is formed. The oxide layer at the interface 24 of the members 20, 22 extending through the joint 26 defines a weak portion of the joint 26 that is subject to failure during use.
In addition, friction stir welding of lap joints often results in a “thinning” of one or both of the structural members 20, 22 adjacent to the weld joint 26. Thinning generally refers to a condition in which the interface 24 between the members 20, 22 of a workpiece is deformed, i.e., either lifted or lowered toward the outer surface of a respective one of the members 20, 22. The thickness of the respective member 20, 22, as measured between the interface 24 and the outer surface of the member, is reduced, and the member 20, 22 is therefore subject to an increased risk of failure when stressed.
Conventional friction stir welding tools and methods have been shown to form weld joints in which the material of the workpiece is plasticized and mixed, and the resulting grain structure in the weld joint is refined. However, a need continues to exist for an improved friction stir weld tool and method for sufficiently mixing and refining the workpiece throughout a lap joint. Preferably, the tool and method should be capable of forming a lap joint, in which the joint extends substantially perpendicular through the interface.